Wind turbines are used to convert the kinetic energy that is contained in the air flowing onto the rotor of the wind turbine into electrical energy. Present-day wind turbines are usually variable in speed, wherein the rotational speed of the rotor can be adjusted via the rotor blade pitch angle of at least one rotor blade of the rotor and via the generator torque of the generator of the wind turbine, acting on the rotor. Excessively high wind speeds can damage a wind turbine if its power output and rotational speed are not limited in good time. For this purpose, from a lower limit wind speed onwards, for example, the rotor blades are rotated such that the rotor of the wind turbine absorbs less power from the incident wind, and the rotational speed of the rotor is reduced. Above an upper limit wind speed, a load-free operating mode may be provided for a wind turbine, in which the rotor of the wind turbine can freely rotate at comparatively low rotational speeds without a generator torque acting on it, or the rotor of the wind turbine is moved into a parking position. In the load-free operating mode, the wind turbine does not convert any kinetic energy into electrical energy.
In order to increase the annual energy production of wind turbines, it is desirable to provide a method and a device for operating a wind turbine that enable reliable operation at a maximum power output with respect to the permissible operating loads, even in the case of high wind speeds.
WO 97/09531 A1 relates to a method for operating a wind turbine, in particular for limiting the loading on a wind turbine, preferably on a pitch-controlled wind turbine. The loading on the wind turbine is to be limited in the case of higher wind speeds. This is achieved by reducing the power output of the wind turbine depending on the wind speed, from a preset wind speed onwards, by reducing the rotational speed of the rotor of the wind turbine in case of occurrence of wind speeds above a limit wind speed.
US 2018/0045180 relates to a method for operating a wind turbine in which the rotational speed and the power output of the wind turbine are reduced if the prevailing wind speed exceeds a preset first limit value. The rotational speed and the power output are further reduced as the wind speed increases further, until the rotational speed attains a preset minimum rotational speed and/or the power output attains a preset minimum power output. These are maintained if the wind speed increases yet further.
From prior art various methods are known, in which, above a preset wind speed, the power output and the rotational speed of a wind turbine are reduced. Although the operating loads can thereby be reduced in an effective manner, maximizing of the annual energy yield is not achieved.